Come on. It would be a total lack of sanity to know with certainty Trump won by a landslide, as he claims.
Many people are delusional to some extent, especially politicians. I don't think it's ever been the basis for the 25th.
They believe so because Trump and his allies have been propagating that.
Perhaps yes, perhaps not. When you don't allow for meaningful oversight, that is realistic fodder for people to start thinking that something sinister took place. Maybe it was fueled by Trump's rhetoric, but I don't think it was the only catalyst.
In all seriousness, I don't think anyone can deny that the election was stolen from Trump on the regulation side of things, though it was done in a completely (or almost completely) legal manner. The fact that the pandemic was used to allow for mass mail-in voting, without which it is fair to say that Trump had the win in the bag, is enough to say that the election was stolen from him. Again, it was legal, so there is nothing to do about it. But to say with certainty that there was definitely enough
fraud to overturn the election, I agree is not realistic.